1. Field of the Invention
The present invention relates to a diamond film and a method for producing the same.
2. Related Art
In recent years, diamond films formed on base materials by utilizing unique properties of diamond have been widely investigated. For example, they are investigated as masking members for light exposure in lithography techniques used in the semiconductor device production, substrates for surface acoustic wave (SAW) devices, grinding and polishing tools and so forth.
In recent years, use of diamond films as a masking membrane in the X-ray lithography and electron beam lithography, which enables formation of extremely fine patterns of 100 nm or less, utilizing their properties of high Young's modulus, etching resistance, high-energy ray irradiation resistance and so forth attracts attentions.
As the methods for producing a diamond film, there are known methods of depositing it on a base material by a vapor phase reaction using DC arc discharge, DC glow discharge, combustion flame, radiofrequency wave (R.F.), microwave, hot filament and so forth. Among these production methods, the microwave CVD method and the hot filament CVD method are generally used, since they enable formation of films having a large size and good crystallinity.
Meanwhile, a raw material gas used in the aforementioned methods of depositing on a base material by a vapor phase reaction is generally a mixed gas obtained by diluting a carbon-containing gas such as methane, ethylene, acetylene and carbon monoxide with hydrogen gas. Electric resistivity of a diamond film obtained by performing a vapor phase reaction using such a hydrogen-diluted carbon-containing gas as a raw material gas is in the range of 109 to 1015 Ω·cm.
Further, when a diamond film showing an electric resistivity value in such a range is used, for example, as a mask for lithography, especially a mask for X-ray or electron beam lithography, a defect test of the mask must be performed by irradiation with an electron beam. But the resistivity is too high for the test, thus a charge up phenomenon is likely to occur due to accumulation of charged particles, and therefore such a defect test cannot be carried out quickly and precisely in many cases.
Moreover, when it is actually used as a mask for electron beam lithography, a high resistivity causes a problem in image transfer due to the charge up.
Therefore, in order to avoid such a charge up phenomenon, it has been proposed that a dopant gas such as diborane (B2H6) or phosphine (PH3) is mixed with the aforementioned hydrogen-diluted carbon-containing gas during the production of a diamond film to perform a vapor phase reaction and thereby reduce the electric resistivity value of the diamond film.
Specifically, it is reported that, if B2H6 as a source of boron to be doped and hydrogen-diluted methane gas as a raw material gas are mixed, and then such a mixed raw material gas is introduced into a chamber to perform a vapor phase reaction and thereby produce a diamond film of P-type, the electric resistivity value of the diamond film can be decreased to 10−2 Ω·cm (K. Marumoto, J. Appl. Phys., 31 (1992) 4205–4209).
However, if B2H6 is used as the source of boron to be doped as described above, influence on human bodies is feared, since the permissible concentration thereof is 0.1 ppm. Further, when PH3 is used as the source of phosphorus to be doped, influence on human bodies is similarly feared, since the permissible concentration thereof is 0.3 ppm. That is, leakages even in a small amount are not accepted in doping using these gases. Furthermore, since they not only influence on human bodies but also show explosiveness, and hence they have also a problem concerning handling. Therefore, in order to use these gases, special apparatuses for safety must be used for the apparatuses themselves, piping and so forth, and thus increase of cost is unavoidable.
With the purpose of obviating these problems, there has also been proposed a method of adding boron by vaporizing a solution dissolving B2O3 in methanol, ethanol, acetone or the like and mixing it with a hydrogen diluted raw material gas for use as a mixed raw material gas. However, this technique has a problem that it is difficult to produce a uniform solution or control temperature of the solution, and thus it is difficult to obtain a uniform diamond film with good reproducibility.